The present invention is generally related to precision measurements using comparators and, more particularly, to time expansion circuits for pulse counting in time interval meters.
It is frequently necessary to provide precise measurements of time intervals in the sub-microsecond range. In one technique, a capacitor is rapidly charged during an event timing interval and then slowly discharged after the event. Typical time interval meters are illustrated in R. Nutt, "Digital Time Intervalometer," 39 Rev. Sci. Instr. No. 9, pp. 1342-1345 (Sept. 1968) and U.S. Pat. No. 4,613,950, "Self-Calibrating Time Interval Meter," issued Sept. 23, 1986, to D. Knierim et al., incorporated herein by reference.
In a time interval expansion-type meter, a capacitor is rapidly charged during a time interval to be measured and is thereafter discharged at a much slower rate after the event time interval. The capacitor discharges until it reaches a reference potential, typically ground potential, whereupon a comparator indicates the end of the slow ramp period. A precision clock source is typically included wherein the clock pulses are counted during the capacitor discharging for precise timing. The number of clock pulses counted during the capacitor discharge ramp are proportional to the duration of the discharge, which, in turn, has a known relationship with the capacitor charge ramp so that the event interval becomes known with precision.
Clock pulses are counted during the interval between the start of the discharge ramp and the time the discharge voltage equals a reference voltage. However, active circuit components must have a high bandwidth to accommodate the capacitor discharge/charge rates. Such components are generally temperature sensitive and the reference voltages which define the counting interval can drift. Capacitive coupling might be used to eliminate this dc drift, but such a circuit would have application only to events with very low repetition rates, with a concomitant increase in testing time. Circuit components having a limited bandwidth could also be used, but there would be a resulting nonlinearity.
It would be desirable to provide a ramp timing circuit which could define reference voltages for the timing interval which would enable the timing interval to be independent of voltage drift values.
This problem is addressed by the present invention, wherein a circuit is provided to establish a reference interval voltage as a function of the ramp start voltage so that equal capacitor discharge/charge voltage swings result in equal time interval measurements. Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.